2006 Honda Civic General Question
 

Vehicle threw an error code of P0128 Engine Coolant Temperature Sensor. Seems pretty straight forward. There are 2 sensors (upper and lower). Any advice on what to do and not to do would be great.

A few other items:

Vehicle has 140,000 miles on and has a slight rough idle:

Car History:

1) Plugs will be changed as they never have been
2) Coolant will be changed as it has never been serviced
3) Tranny Fluid is dark, but not burned and has never been changed - SHOULD THE TRANSMISSION FLUID BE EVEN BOTHERED WITH? I have heard that if never done, do not touch. Is there any risk? Thoughts?

Does this car have a fly by wire throttle body, and what steps should be taken to clean, etc. Just added a bottle of Techron Complete Fuel System Cleaner. Thanks!

PS: This is mom's vehicle.